The invention relates to an optical relay having a target of electrically insulating material which allows light to pass in a manner depending on the electric field parallel to the direction of propagation of this light. An electron beam scans a first face of this target and an anode receives the secondary electrons transmitted in response to the action of the beam. An optically transparent and electrically conducting plate is provided against the second face of the target to receive the electric signal conveying the video information and thus forming the control electrode. The target is made of a material which becomes ferro-electric below a given temperature, the Curie temperature. A heat exchanger is connected to a heat source which maintains the temperature necessary for its operation and acts on the target. The target is bonded to a plate of an adequately heat conducting material, so that during the normal mode of operation the target is formed by:
a useful central zone where it exhibits a single-domain ferro-electric phase, PA0 and a neutral peripheral zone having a temperature higher than the Curie temperature, where it exhibits a paraelectric phase. PA0 f1--it produces the energy which is to appear in the form of light: the light output of the tube is consequently always less than the power transferred by the beam; PA0 f2--it scans the surface of the picture; PA0 f3--it provides the visual information. PA0 piezo-electric coefficients PA0 electro-optic coefficients PA0 dielectric constants .epsilon..sub.x and .epsilon..sub.z. PA0 a useful central zone where it exhibits a monodomain ferroelectric phase. PA0 and a neutral peripheral zone, having a temperature much higher than the Curie temperature, where a paraelectric phase is present.
An optical relay of this type for projecting televison pictures, is disclosed in the French patent specification Nos. 1,473,212 and 1,479,284. For a more detailed characterization of the invention the operating principle of this optical relay follows hereafter. More details can be found in the above documents.
The framework of the invention relates to the conversion of a time-variable electric signal which represents the video information, into a visible picture. This is one of the functions of a televison receiver.
In the "picture" tube of such a receiver the electron beam conventionally performs the three fundamental functions of this conversion:
Because of, inter alia, the functions f2 and f3, the beam power and hence the picture brightness cannot be raised to an extent as is required for projection on, for example, a large screen.
It has therefore been proposed to separate these functions and to have the function f1 performed by, for example, an arc lamp, the functions f2 and f3 by what is commonly referred to as an "optical relay". Such a relay uses a crystal exhibiting an electro-optical effect, denoted the "Pockels" effect. A crystal of double acid potassium phosphate KH.sub.2 PO.sub.4 termed KDP hereinafter, is suitable for this purpose.
According to the Pockels effect, when an electrically insulating crystal is exposed to an electric field parallel to its crystal axis c (the three crystal axes a, b and c form a trieder of three rectangles, in this case the axis c being the optical axis), the index n of this crystal for light rays propagating in the direction c and being linearly polarized in the ab-plane, depends on the direction of this polarization. Put more accurately, if X and Y designate the bisectors of the axes a and b, and if the parameters of the crystal with respect to these different directions are designated by the letters used for these directions, the diagram of the indices in the abplane becomes an ellipse having X and Y axes instead of becoming a circle, and that difference (n.sub.x -n.sub.y) is proportional to the applied electric field. It follows that if the incident light rays are polarized parallel to the axis a, the luminous intensity I passing through an output polarizer will be i=I.sub.o sin.sup.2 kV if the direction of polarization of this polarizer is parallel to the axis b, and I=I.sub.o cos.sup.2 kV if the direction is parallel to the axis a. I.sub.o is the intensity of the incident light if no parasitic absorption occurs, V is the electrical potential difference between the two faces of the crystal, and k is a coefficient depending on the crystal material used.
To obtain a picture, by projection by means of a lamp via this assembly, it is sufficient, to apply an electric field parallel to the axis c and to cause the value of this field at any point of the plate to correspond to the brightness at the corresponding point of the picture to be obtained. For this purpose, an electron beam produced by an electron gun and passing through conventional deflection members, scans the target, thus performing the function f2. As for the function f3, that is to say here the control of the electric field, this function is also performed by the beam in the following manner.
When the electrons of the beam strike the target surface, they cause, if their energy is comprised between appropriate limits and to the extent in which the potential of the anode is sufficiently high, the emission of secondary electrons in a quantity exceeding the quantity of the incident electrons. This results in the electric potential of the point reached being increased, so that the potential difference between the anode and this point decreases. If the electrons of the beam reach this point in an adequate quantity, this potential difference becomes negative and reaches a value such (-3V for example) that each incident electron no longer causes the emission of one single secondary electron. The potential in this point is thus fixed at a limited value relative to that of the anode. In this respect it is sufficient, taking account of the scanning rate, that the beam intensity is adequate. The anode potential being constant, each passage of the electron beam fixes, as has already been described in the foregoing, the potential at any point A of the surface at a value V.sub.0, independent of this point and the instant of passage, However, the corresponding electric charge appearing in this point depends on the potential of the nearby control electrode, at the other side of the target.
If the potential of this electrode at the instant of passage is denoted VA, this charge is proportional to VO-VA, VA representing the value of the video information signal at the instant of its passage.
The target whose double refraction depends on the electric field, is constituted by a single crystal of KDP, in which approximately 95% of the hydrogen is formed by heavy hydrogen (deuterium).
The Pockels effect is proportional, for a given crystal width, to the charges appearing on the crystal surfaces and consequently, for a given control voltage, is proportional to the dielectric constant thereof. For that reason a target is used which is constituted by a crystal which becomes ferroelectric below a certain temperature, the Curie temperature, and advantageously the crystal is operated near this temperature, as then the dielectric constant reaches very high values and the optical relay can function by means of control voltages which are easy to handle (the Pockels effect being proportional to the product .epsilon.V).
The most frequently used crystals exhibiting this phenomenon are acid salts, particularly of the KDP-type in the class of the quadratic crystals, the optical axis of which is parallel to the crystal axis c. Its Curie temperature is located near -53.degree. C. Above the Curie temperature, the DKDP is a quadratic crystal of the symmetry class 42 m and it has a paraelectric behaviour. Below the Curie temperature the DKDP becomes orthorhombic, symmetry class mm2, and it exhibits a ferroelectric behaviour: locally there is spontantaneous polarization and the appearance of ferroelectric domains.
At the ambiant temperature, the crystal is anisotropic but in the proximity of the Curie point the anisotropy becomes extremely important. The change of state is accompanied by abrupt variations of the physical properties, along the crystal axis:
Thus, the dielectric constant .epsilon..sub.z changes from a value of approximately 60 at the ambiant temperature to a value of 30,000 at the Curie temperature.
It is known, that, from the electro-optic point of view the apparent width e of the DKDP crystal is EQU e=1(.epsilon..sub.x /.epsilon.'.sub.z).sup.0,5.
The target appears to be thinner according as the ratio .epsilon..sub.x /.epsilon..sub.z is smaller wherein .epsilon.'.sub.z is the value of .epsilon..sub.z when the crystal is mechanically blocked. Actually, in an optical relay, the monocrystalline DKDP sheet having a thickness l near 250 microns, is firmly cemented to a rigid support: a fluorine sheet 5 mm thick.
The target of the optical relay is then usually cooled to -51.degree. C., that is to say to a temperature slightly above the Curie point. In these conditions .epsilon..sub.x /.epsilon.'.sub.z =1/9 and the apparent thickness of the crystal is approximately 80 microns, which gives the optical relay a good image resolution. Below the Curie point, this ratio .epsilon..sub.x /.epsilon.'.sub.z is still significantly lower, which much improves the image resolution.
Up to the present it has not been possible to utilize for the projection of televized pictures, a target cooled to below its Curie temperature. Actually, the change of state causes the systematic appearance of ferroelectric domains which on the projection screen are apparent from the display of a large number of bright vertical and horizontal lines which are distributed in a disorderly manner across the picture. These domains correspond to zones having a different atomic arrangement.
In a patent application No. 85 13 989 filed on Sept. 20, 1985 and which has not yet been published, it has been proposed to have the optical relay operate just below the Curie temperature of the target, with a final picture which does not show any deterioration due to bright disordered lines.
With the object of keeping the target at the temperature necessary for its operation, it has been proposed to provide the optical relay with a heat exchanger which acts on the target such that during normal operation of the target there are formed:
When the ratios .epsilon..sub.x /.epsilon.'.sub.z in the paraelectric phase and in the ferroelectric phase are compared, it will indeed be found that it is possible to improve the intrinsic resolution of the target by having it operate in the ferroelectric state.
In said application the importance of operating at a temperature below the Curie point is demonstrated, the ratio .epsilon..sub.x /.epsilon.'.sub.z then becoming much lower, which distinctly improves the picture resolution of the optical relay. Typically at the spatial frequency of 1000 picture elements per line and at the nominal beam current of 60 .mu.A, the contrast which is 11% at the usual operating temperature (-51.degree. C.), reaches 24% when the temperature of the target is below the Curie point and that more specifically over a range of approximately 10 degrees (from -63.degree. C. to -53.degree. C.).
Said application has proposed to replace the centripetal cooling of the target by a centrifugal cooling.
In accordance with the normal mode of construction, the KDP target, which is rectangular, is cemented to a fluorine sheet, which is a good heat conductor. This fluorine sheet is set in a copper frame which has for its object to transfer the negative kilocalories, this frame being mounted on Peltier-effect, refrigerating elements. Thus, the target is cooled centripetally: the corners of the KDP target reach the Curie temperature first, thereafter the sides; a "cold circle" appears which defines the boundary between the two states, paraelectric in the centre and ferroelectric at the periphery. Simultaneously, in the peripheral zone which has now become ferroelectric, bright demarcation lines appear which were already found between the multiple ferroelectric domains. These domains are first microscopically small, but inevitably deteriorate the edges of the KDP target.
Said application has proposed to have the cold move progressively from the centre of the target and to prevent the cold zone from reaching the edge of the target, so as to provide that the crystal becomes ferroelectric in the centre and paraelectric at the periphery. The centre, which is the useful zone for the projection of the picture, is then ferroelectric and monodomain. This state remains stable whilst the ferroelectric central zone remains surrounded by a paraelectric peripheral zone.
Embodiments which cool the target by means of its center safeguard the optical properties of the center for the projection of the picture, but exhibit an inadequate solidity and reliability. In addition, the control of the temperature deviation between the ferroelectric zone and the paraelectric zone is very difficult to realize and is for a major part defined by the manufacture of the substrate. On the other hand the proposed embodiment is not suitable for obtaining a useful zone of rectangular shapes without a loss in useful target surface area.